Spiral separation membrane elements used for, for example, seawater desalination and ultrapure water production, are conventionally known (see, for example, Patent Literature 1). Generally, in a spiral separation membrane element, a layered body including a separation membrane is wound around a central tube, and end members are disposed on both sides of the layered body. The end members serve, for example, to prevent the layered body from extending telescopically.
A feed liquid fed to the spiral separation membrane element passes through one of the end members and then is separated into a permeate liquid and a concentrated liquid by the separation membrane. The permeate liquid is discharged through the central tube, and the concentrated liquid is discharged through the other end member.
The end members have a configuration in which an inner annular portion and an outer annular portion are connected together by a plurality of ribs. The use of twisted shape ribs in these end members have been proposed to form a swirling flow in the feed liquid (see, for example, Patent Literature 2).